Calculation of the temperature dependence of the thermal emf in amorphous alloys CaxAl1−x and AuxNi1−x

The temperature dependence of the thermal emf of Ca_xAl_1–x and Au_xNi_1–x for four different concentrations of the components of the alloys is calculated on the basis of the concept of dynamic concentrated excitations in amorphous metal systems. It is shown that increasing x from 0.15 to 0.50 in Au_xNi_1–x raises the thermal emf, and a further increase in the Au concentration from 0.50 to 0.80 lowers S(T). For Ca_xAl_1–x the dependence S(T) is calculated in the interval of Ca concentrations from 0.55 to 0.75. In this concentration interval the thermal emf decreases as x is increased. It is shown that for both types of alloys the S(T) curve bends abruptly at a temperature near 10T₀ (where T₀ is the concentration-dependent characteristic temperature of amorphous alloys separating the ranges of strong and weak scattering of electrons by dynamic concentration excitations). The so-called S(T) knee shifts toward lower temperatures when the thermal emf increases with increasing x and toward higher temperatures when S(T) decreases with increasing x. The results agree with experimental data.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1994.     

EPR study on the La2/3Ca1/3Mn1−x Cu x O y system

The interaction between Mn and Cu ions is studied by measuring the resistance and electron paramagnetic resonance (EPR) at different temperatures of the Cu-doped compound La_2/3Ca_1/3Mn_1−xCu_xO_y. A new transition inR-T curve and substantial enhancement in magnetoresistance are induced by the substitution of Mn ions by Cu ions. The EPR measurement shows that two resonance signals appear at temperature lower than the spin-ordering temperature of Mn ions. A tentative interpretation for the observed phenomena is proposed by considering the interaction between the Cu/Mn ions besides the Mn³⁺/Mn⁴⁺ ions.     

Surface effect on the reverse current of Al x Ga1−x Sbp−n structures

The effect of an invertedp-region along the free surface ofn-Al _x Ga_1−x Sb on the reverse current ofp−n structures from the given solid solution is analyzed. Expressions which describe “collection” of the inverted layer current on the cylindrical surface of ann-region are discussed. The contribution of the near-surface and bulk components to the reverse current ofp−n structures with a semi-infiniten-region is estimated. For structures with a two-layern-region of finite thickness we have calculated the dependence of the near-surface current on the voltage across thep−n structure, the thickness of then-region, and its composition and doping level. We have compared the calculated current-voltage characteristics with experiment using a Al_0.15Ga_0.85Sbp−n structure as an example. Tomsk State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 34–40, January, 1999.     

The effects of random fields on the critical and bicritical behavior of Mn x Zn1−x F2

We have carried out a comprehensive study of the static and dynamic spin-spin correlations of Mn _x Zn_1–x F₂ in a magnetic field. Samples withx=0.75 andx=0.5 have been studied. This system exhibits behavior closely related, if not identical, to that of the Random Field Ising Model (RFIM). An additional feature of Mn _x Zn_1–x F₂ is that it exhibits an easily accessible bicritical point; thus one can study the changeover from the RFIM to the uniformXY model with a transverse random field. Quite generally, the instantaneous spin-spin correlations in a field are described by a combination of Lorentzian, Lorentzian-squared and delta function terms the latter corresponds to the long range order (LRO) component. In the Ising phase one finds history dependent behavior as discussed previously. In theXY phase, except very near the spin-flop boundary, one finds ergodic behavior withXY LRO and Lorentzian squared Ising fluctuations. Rather complicated instability effects are found all along the spin-flop boundary. Further, when one establishes LRO in theXY phase and lowers the field through the spin-flop value, one obtains a LRO Ising state in thex=0.75 sample whereas one obtains the field-cooled domain state in thex=0.50 sample. This dramatic difference in behavior is not understood. Our results on the RFIM aspects of the problem are consistent with our previous studies. The transition is dominated by the metastability effects with an underlying equilibrium transition which is either first order or weakly second order (0). The underlying transition manifests itself directly in measurements of the dynamic response nearT _N (H). From the data above the metastability boundary we deduce for the static correlation length exponentv=1.4±0.3 in good agreement with theory. We find for the RFIM crossover exponent _RF=1.5±0.2 where the errors represent the spread in values obtained from different techniques. Finally, we have determined in detail the field-temperature phase diagram of thex=0.5 sample including the critical behavior along the spin-flop line; the latter transition appears to be second order for an extended region.     

Low temperature photoluminescence properties of p- and n-type Al x Ga1−x As withx>0.42

The photoluminescence (PL) spectra of n- and p-type Al _x Ga_1−x As (x>0.42) grown by metalorganic vapor phase epitaxy (MOVPE) and liquid phase epitaxy show typically a broad PL band (BB) centered about 300meV below the near band-gap PL lines. In the MOVPE grown samples the BB is composed out of four lines. The BB intensity increases with the doping level and dominates the spectrum at concentrations > 10¹⁷ cm⁻³. The temperature dependence of the BB intensity shows two distinct maxima at ≈ 19K and ≈ 80K. Hydrogenation of MOVPE grown samples at 170°C reveal an effective passivation of the shallow acceptors and the centers associated with the BB line. On the contrary hydrogenation at temperatures >250°C leads to an increase in the BB intensity which is related with the evaporation of As from the surface and the generation of V_As in the layers at these temperatures. Our results suggest that the BB is related with isoelectronic centers formed when V_As and C_As or Si_As associate to form next nearest neighbor ion pairs. The PL is then due to a recombination of excitons in analogy to the case of (Zn, O) isoelectronic complexes in GaP. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was supported by the DFG under the contract AZ 436 TSE 113/22/0 in the framework of the cooperation between the DFG and the Academy of Sciences of the Czech Republic.     

Variations of phase transition temperature and enthalpy in the systems Na2Mo1− x W x O4 and Na2Mo1− y S y O4

The samples in two material systems, Na₂Mo_{1? x} W _x O₄ and Na₂Mo_{1? y} S _y O₄, were prepared by using conventional solid reactions and characterized by X-ray diffractometer (XRD), energy dispersive X-ray spectroscopy (EDX), and difference scanning calorimetry (DSC). The XRD and EDX data indicated that all the samples in both systems were in the solid solution range. The DSC data indicated that in the system Na₂Mo_{1? x} W _x O₄, the solid–solid transition temperatures increased and in the system Na₂Mo_{1? y} S _y O₄, the solid–solid transition temperatures decreased. The total enthalpy (ΔH _total) of the solid–solid transitions in the system Na₂Mo_{1? x} W _x O₄, decreased much less than that in the system Na₂Mo_{1? y} S _y O₄. This is probably because similar to Na₂MoO₄, the solid–solid transition of Na₂WO₄ has relatively large ΔH _total, but Na₂SO₄ has much smaller solid–solid transition enthalpy. In order to modify the transition temperatures of Na₂MoO₄ and also to keep its relatively large ΔH _total, it is necessary to choose a doping material with large ΔH _total.     

Negatron effects in CdSe1 − x Te x and ZnS1−x Se x films

Various negatron effects in films of alloys of II–VI compounds deposited from solutions as a function of the deposition mode and heat treatment are studied. It is found that the negative photocapacitance effect, which was first discovered in ZnS_1?x Se _x films, and the slowly relaxing negative photoelectric effects, which are caused by the transition of electrons located in a nanoscale surface layer from the shallow energy levels of trapping centers to deeper levels with a lower polarizability and by the presence of nanoscale clusters in these materials, which play the role of a “reservoir” for minority charge carriers, occur according to a single mechanism. A model to explain the basic laws of negative photoconductivity in CdSe_{1 ? x} Te _x films deposited from a solution is proposed. Negative residual conductivity is explained in terms of double-barrier relief model, while negative differential photoconductivity is attributed to the presence of nanoscale electric domains.     

The preparation,crystallography, and magnetic properties of the LixCo(1−x)O system

The Li_xCo(_1−x)O system has been prepared by the solid-state reaction between Li₂O₂ and CoO at 900°C. The material is a single-phase structure in the composition range 0 < x < 0.2, the lattice parameter of the cubic unit cell decreasing with increasing lithium content. At the composition x = 0.5 (i.e. LiCoO₂) a rhombohedral structure is formed. A phase diagram for this system has been developed.The crystal structure of material in the composition range 0 < x < 0.2 has been studied both above and below the antiferromagnetic Curie temperature. Both the antiferromagnetic Curie temperature and the related cubic-tetragonal transformation are found to occur at decreasing temperatures with increasing lithium content. In addition the magnitude of the crystallographic deformation decreases and finally vanishes with increasing lithium content.The magnetic-susceptibility data indicate that the magnetic moment of Co⁺² is best understood if the multiplet splitting is small compared to kT. Further, the moment of Co⁺³ is shown to be lowered by crystalline field effects. No effects attributable to the double exchange coupling were found. The absence of such a coupling is understood by consideration of the mechanism of electron transfer in the lithium-substituted compounds.     

The Fe1−xCoxS system (x < 0.25); transition and the high temperature phase

The α-transition in Fe_1−xCo_xS materials is examined based on electrical, magnetic and structural properties. A decrease of the transition temperature is observed with increasing alloying rate up to x_co = 0.24, where the transition is completely cancelled. Beyond this critical concentration the materials exhibit a high-temperature FeS type behavior.The existence of hk0 reflections, with h and/or k = 2n + 1 in the cobalt-stabilized superstructure, the high-temperature phase of FeS, establishes that the structure is hexagonal, space group P6₃mc, with two independent metal positions, in contrast to the previously proposed tri-twinned MnP type model.The physical and structural results are discussed within the framework of conductivity by polarons. The α-transition occurs when the polaron concentration, thermally created or introduced by impurities, reaches the critical value x_p ≅ 0.08. The driving mechanism for the α-transition is a reduction of the polaron dissociation energy by a temperature increase or by cobalt alloying.     

Laser radiation of Cd x Zn1−x S semiconductor targets of the gas diode

The experimental results on the study of radiation of Cd _x Zn_1?x S semiconductor targets (STs) of the gas diode (GD) for the pressure variation from 10^?1 Torr to the atmospheric pressure are presented. Pulses 0.5–1 ns long with an amplitude to 200 kV were applied to the GD cathode. Laser radiation (509 nm) was generated in the ST under a beam of accelerated runaway electrons to a pressure of 2.5 Torr. At atmospheric pressure, generation in the ST was observed in discharge channels when the streamer moved from one ST surface to another. In this case, as the electric fields strength increased, radiation sequentially arose at three spectral lines, 509, 480, and 469 nm. Possible causes of the observed phenomena are considered.     

57Fe Mössbauer and infrared studies of the system Co1−x Cd x Fe2O4

Mössbauer and infrared studies were made on samples of the ferrite system Co_1–xCd_xFe₂O₄ x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1. Mössbauer spectra were taken at room temperature. The spectra of the samples withx0.7 showed well defined Zeeman patterns and they have been analyzed with two components, one due to A-site Fe³⁺ ions, and the other due to B-site Fe³⁺ and Fe²⁺ ions. The pattern due to B-site appeared to be composite and an explanation is given. The spectra withx=0.9 and 1 showed only a quadrupole splitting. The effect of cadmium substitution on the various hyperfine interactions has been discussed and the cationic distribution has been deduced for all values ofx. Far infrared spectra of the ferrite samples in the range 200–700 cm^–1 were reported. Four bands were observed: the high frequency bandv ₁ is assigned to tetrahedral complexes, and the low frequency bandv ₂ to octahedral complexes, a small bandv ₃ is due to Co²⁺-O^2– complexes andv ₄ is assigned to the lattice vibration of the system. The splitting occurred in thev ₁ andv ₂ bands atx=0.9 and inv ₂ atx=1, indicating the presence of Fe²⁺ ions in octahedral sites.     

Studying the elemental composition and manganese distribution in Zn1 − x Mn x Te and Zn1 − x Mn x S films using the μ-PIXE and EDAX methods

The methods of proton-induced X-ray emission (μ-PIXE) and energy-dispersive X-ray analysis (EDAX) are used to study the elemental composition and areal distribution of manganese in Zn_{1 ? x} Mn _x Te and Zn_{1 ? x} Mn _x S films. The μ-PIXE technique is implemented using a nuclear scanning microprobe with a proton-beam energy of 1.5 MeV and cross-sectional dimensions of 4 × 4 μm. The semimagnetic solid solution films are obtained under different operating modes of thermal vapor deposition in the quasiclosed volume of a manganese-containing blend.     

Experimental and theoretical investigations of the Knight shift of Pd and Ag in the alloy system Ag x Pd1−x

The Knight shift of Pd in Ag _x Pd_1–x has been determined for concentrationsx0.2. In full accordance with the expectations based on the behaviour of the magnetic susceptibility, it was found that the Knight shift of Pd is rapidly reduced in magnitude by adding Ag to Pd. To allow for a detailed interpretation of this finding, we have performed Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure calculations for Ag _x Pd_1–x . These calculations clearly demonstrate that the decrease in spin susceptibility with increasingx is accompanied with a decrease in core polarization. In contrast to Pd, the negative Knight shift of Ag on the Pd-rich side of the system is caused by the valence band contribution, as it is demonstrated by our calculations. This is caused by an intersite effect in analogy to the transferred hyperfine field found for non-magnetic elements dissolved in a magnetic host.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

Effect of nanostructuring of the Ge1 − x Mn x single-crystal alloy on the percolation and cluster ferromagnetism

Nanostructuring of the Ge_0.99Mn_0.01 single-crystal alloy with the formation of nanowires 60 nm in diameter has been revealed to bring about an increase in the temperature of the percolation transition to the ferromagnetic state to T _C1 ≈ 45 K and a broadening of the distribution of Curie temperatures in Ge _n Mn _m clusters T _C2 ≈ 125?270 K as compared to Ge_0.98Mn_0.02 thin single-crystal films for which T _C1 ≈ 6 K and T _C2 ≈ 270 K.     

Effect of temperature on the band structure of MnxHg1−xSe

The band structure of the solid solutions Mn_xHg_1–xSe is constructed on the basis of the band parameters obtained from the electrophysical and optical investigations of these solid solutions. It is shown that the band structure of Mn_xHg_1–xSe and the dynamics of the bands as a function of the composition and temperature are analogous to those for Cd_xHg_1–xTe. It is shown that the variation of the nonparabolicity of the band spectrum of Mn_xHg_1–xSe as a function of the temperature greatly affects the temperature coefficient of the change of the width of the optical forbidden band (d_gop/dT).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 40–44, April, 1991.     

Dielectric relaxation in LaSrCo1−x Al x O4 ceramics

Dielectric properties of LaSrCo_1−x Al _x O₄ (x=0, 0.1, 0.3, and 0.5) ceramics were investigated in a broad frequency and temperature range. The AC conductivity decreased with the increasing Al concentration. Dielectric constant increased at lower frequency and decreased at higher frequency when the Al concentration increased from 0.1 to 0.3, then it decreased at all frequencies as the x value was 0.5. While the dielectric loss decreased first and then increased with the increasing Al concentration. There was one dielectric relaxation in the curve of temperature dependence of dielectric properties of LaSrCo_0.7Al_0.3O₄ ceramics. The nonadiabatic small polaronic hopping process should contribute to the dielectric relaxation in the present ceramics. The AC conductivity increased in about one order of magnitude after annealing the sample in the oxygen atmosphere, and this should be attributed to the appearance of interstitial oxygen in the annealed sample.     

Influence of temperature and Pr-doping on the dielectric properties of Ca2−xPrxMnO4 compounds and structural transition effects

Complex perovskite oxides Ca_2?xPr_xMnO₄ (x = 0–0.5) compounds were synthesized by a solid-state reaction technique. A tetra–ortho structural transition was observed. Impedance spectroscopy was used to study the electrical behavior in the frequency range 40 Hz–1 MHz and in the temperature range 80–350 K. Frequency-dependent conductivity spectra were found to obey the Jonscher's power law. Complex impedance plane plots have indicated that the dielectric response is mainly intrinsic. Materials bulk response was found to be dominated by non-localized or localized conduction, depending on temperature and frequency.